1. Field of the Invention
This invention relates to electro-optic voltage measuring apparatus in which a shift in phase of polarized light passed through a crystal exhibiting birefrigance in proportion to an applied electric field provides a measure of the voltage producing the field. More particularly, the invention relates to such apparatus having single ended optics.
2. Background Information
Electro-optical systems for measuring electric voltages are known. For instance, devices known as Pockel cells utilize certain crystals which exhibit birefringence, that is a difference in the index of refraction in two orthogonal planes, in the presence of an electric field. Some of these crystals, such as, for example, KDP (potassium dihydrogen phosphate), have a fourfold axis of rotary inversion. Such materials have the property that in the absence of an electric field the index of refraction for light propagating along the fourfold axis is independent of the plane of polarization of the light. However, if an electric field is applied parallel to the direction of the light, the index of refraction for light polarized in one direction transverse to the fourfold axis, known as the fast axis decreases and that in an orthogonal direction, also transverse to the fourfold axis, and known as the slow axis, increases by an amount which is proportional to the strength of the electric field. In such Pockel cell devices, if light is polarized in a plane which forms an angle to these transverse axes, the component of the polarized light in the direction of the slow axis with the increased index of refraction is retarded with respect to the other component. If the crystal is aligned with its fourfold axis extending between the objects between which the voltage is to be measured, and the polarized light is directed parallel to the fourfold axis, the total retardation will be proportional to the total voltage differential between the two objects. This retardation is typically measured in wavelengths. The retardation is detected in an analyzer and converted to an electrical signal for producing an output representative of the magnitude of the voltage generating the field. Due to the cylic nature of this electrical signal, the output is only unambiguous for voltages producing a retardation which is less than the halfwave voltage for the crystal. In KDP, this halfwave voltage is about 11300 volts.
One application for electro-optic voltage measuring systems is in measuring the voltages of several hundred thousand volts in electric power transmission and distribution systems. Obviously, devices which can only measure voltages up to a halfwave voltage of a little more than 10,000 volts are not suitable for this purpose. U.S. Pat. No. 4,904,931 discloses an electro-optic voltage measuring system in which two polarized beams of light are passed through an electro-optic crystal having a fourfold axis of rotary inversion extending between electrodes carrying the voltage to be measured. The components parallel to the fast and slow axes of each of the polarized beams of light are shifted in phase in proportion to the magnitude of the applied voltage. In addition, the components of one of these polarized beams are additionally retarded with respect to the components of the other polarized light beam by a 1/4 wave plate, or by two 1/8 wave plates with their axes rotated 90.degree. with respect to each other, one in the path of each polarized beam. These two polarized light beams with a difference in phase of 1/4 wave between their respective fast and slow components, are converted into a pair of electrical signals in quadrature. A digital computer calculates from these quadrature electrical signals an unambiguous representation of the instantaneous values of applied voltages including voltages in excess of the halfwave voltage of the crystal.
In the voltage measuring system described in U.S. Pat. No. 4,904,931, the electro-optical crystal is electrically connected at opposite ends to the points between which the voltage is to be measured, for instance between a transmission line and ground. The polarized light is injected into one end of the crystal and analyzed at the other end. Thus, optic fibers are connected to both ends of the crystal. The fibers connected to one end of the crystal must therefore pass from a region at line voltage to ground when the line to ground voltage is to be measured. Because of the limited space available, this is difficult to accomplish without affecting the integrity of the insulation. Furthermore, the electro-optic voltage transducer of U.S. Pat. No. 4,904,931, the electro-optic crystal is supported at both ends.
It is an overall object of the present invention to provide an improved electro-optic voltage transducer.
It is a primary object of the invention to provide such an improved electro-optic voltage transducer in which optic fibers do not have to extend between regions of high voltage differential.
It is a particular object of the invention to provide such an improved electro-optic transducer for measuring a voltage to ground in which all the optic fibers are connected in regions at ground potential.
It is also an object of the invention to provide an electro-optic voltage transducer of the type in which the voltage to be measured is applied to opposite ends of an electro-optic crystal, but in which the crystal is only supported at one end.
It is another object of the invention to provide such a transducer in which the optic fibers are all connected to the one supported end of the electro-optic crystal.
It is yet another overall object of the invention to provide such a transducer which is easier to manufacture and assemble than present transducers and has better electrical isolation.